Owada Communications Unit Intercepts Potsdam Declaration

The Owada Communications Unit was the only Imperial Japanese Navy communications unit whose principal task was the interception of foreign traffic. Its main station lay around Owada-machi, Saitama Prefecture (now Niiza City), with antenna fields spread over an area about 1 km east-west and more than 2 km north-south, while a sub-station in Kiyose handled direction finding. It became independent of the Tokyo Communications Unit in 1941; Japanese Wikipedia credits it with 23 large and about 200 small receivers.

On July 26, 1945, the Potsdam Declaration was issued by the United States, United Kingdom and China and broadcast by the U.S. Office of War Information from San Francisco, relayed via shortwave transmitters on the West Coast, Honolulu, and medium-wave from Saipan. According to the Japanese Wikipedia article on the declaration, it was picked up in Japan early on the morning of 27 July (Japan time) by the Foreign Ministry's radio room, the Domei News Agency and Army and Navy receiving stations, first in English and then in a Japanese-language broadcast. The Japanese Wikipedia article on the Owada site states that the unit received the Potsdam Declaration, without citing a source; the earlier text of this record described it as among the first Japanese military facilities to receive the full text, which could not be verified further in the material consulted, and the order in which the various stations received the broadcast is not known.

The same article says that Owada received the 'Tora Tora Tora' signal reporting the success of the surprise attack on Pearl Harbor. The Japanese Wikipedia article on the attack, however, records that the signal was received directly by the flagship Nagato in Hiroshima Bay and in Tokyo, and the 'Niitakayama nobore 1208' order was transmitted from the Funabashi transmitting station, not Owada. The often-repeated idea that the unit's war was symbolically bookended by receiving 'Tora Tora Tora' at the opening and the Allied ultimatum at the end should therefore be treated as an unsourced tradition.

The declaration was translated and studied by the Foreign Ministry, and the government decided to accept it on 14 August. The Owada facilities were handed over to US forces on 13 September 1945, came under the US Air Force from 1950, and remain in part in use as the US forces' Owada Communication Site. The land in the former Kurume village later became the Higashikurume housing estate and housed the Tokyo Area Control Center from 1963 to 1977; a concrete boundary post of the former Navy land, found during rebuilding, is kept by the Higashikurume City local history room.